An array of dissimilar sensors simulating the human olfactory response has become known as an Electronic Nose [Ref. 1]. An Electronic Nose provides a recognizable visual image in N-dimensional space (where N equals the number of sensors) of specific vapor mixtures (fragrances) containing possibly hundreds of different chemical species. An electronic nose is designed to quantify and characterize all types of smells universally. Sensors are selected for their chemical affinities, and chemi-sorbing polymer films are commonly used for this purpose. Many sensors can be used, and a serial polling of each sensor reading produces a histogram of sensor outputs which comprises the olfactory response of the nose.
An Electronic Nose with only a few sensors results in olfactory responses which are not correlated, and multiple sensors will commonly respond to the same vapor e.g. water vapors. Because of that, it is difficult to calibrate this type of Electronic Nose with test vapors containing more than one compound. Speed and sensitivity also suffer because the vapor sample being tested by the array of individual sensors must be shared equally among all sensors in the array. Additionally there must be sufficient time, typically minutes, for the vapor to be completely absorbed in the chemical coatings.
A more common chemical analysis method of analyzing vapors is to use gas chromatography (Ref 4) to separate the vapor into its individual chemical components. Common GC systems utilize long capillary columns many meters in length, and analysis times are long but accuracy and precision are high. A recent development has been the use of directly heated short chromatography columns, cooled sample traps, and focused surface acoustic wave (SAW) interferometric vapor detectors (Ref. 2).
The SAW detector produces a variable frequency in response to analytes condensing upon and evaporating from the surface of a temperature controlled piezoelectric crystal. Unlike previous GC detectors which measure the flux of the column effluent, the SAW detector measures the total integrated amount of the vapor components as they exit the GC column and condense onto the crystal; it may therefore be referred to as an integrating detector.
Conventional chromatographic data is displayed in a rectilinear format or histogram of detector output signal versus elution time, commonly referred to as a chromatogram. The conventional chromatogram is not well suited to pattern recognition of a visual image, however.
It has been a known practice to utilize a polar display of the outputs from a sensor array. Each sensor output is then arbitrarily assigned a circumferential position in such a display. See References 1 and 3.